The present invention relates generally to communication between devices and more particularly to a method of establishing communication between such devices.
Various devices, significantly digital devices, need to communicate with other similar or dissimilar devices. The purpose of communication may be to transfer, exchange, load or read data between a first device and a second device. In order to communicate, a means of establishing a data link must be provided.
Two types of communication between plural devices have generally been utilized in the art. These two general types of communication are by way of dedicated line and by way of telephone dial-up.
With a dedicated line communication system a number of units or devices are connected to a single communication line (which may be comprised of two or more wires) dedicated for this purpose which is sometimes completed onto itself forming a "loop". Communication between the units can then be established by one unit sending a request over the lines which addresses a second unit with a unique code. The identified unit then answers with an acknowledge and data communication is established. Various schemes have been devised to determine which station, unit or device on the communication line or ring has control for the purpose of sending communication over the line. For the purpose of the present invention the mechanism for deciding which device has control is ignored.
With a telephone dial-up communication system each individual unit or device would have its own telephone line and a unique telephone number. One unit attempting to communicate with a seocnd unit would connect itself to the telphone lines and dial the second unit's telephone number. The second unit would then answer the telphone line to establish communication. Typically, the second or answering unit would respond to the first unit by sending a tone on the telephone line immediately after answering to indicate to the first or originating unit that it is available for data transfer.
A device, called an Intersection Management System (IMS) which is an independent time base controller, is utilized at stoplighted intersections. This unit very accurately keeps track of time in order to properly sequence the traffic signals in order to optimize traffic flow between the through stoplighted intersections. Each IMS unit has a "program" which indicates precisely at what times the stoplight signals at that intersection are to be changed. From time-to-time this "program", really stoplight control data, needs to be changed or replaced. This may be due to a long-term improvement of the traffic flow algorithm or because of a special event, e.g. for locations near a stadium a sporting event, which would drastically changed the normal traffic flow patterns and necessitate a new "program" for those locations.
From time-to-time it is desired to be able to communicate with a particular IMS unit. The purpose of this communication may be to modify the program, to update the time base, or simply to "read" the program or status of the unit. Communication with a particular IMS unit can be accomplished by having personnel physically go to the intersection where each IMS is located and manually modify or retrieve this data. However, it is expensive and time consuming to have personnel physically go to each intersection at which an IMS is located.
Prior art techniques do not work very well in this environment. A dedicated line communication system becomes very expensive and cumbersome due to the number of and length of lines required to be strung or established between all of the IMS unit locations. A telephone dial-up communication system would be much more economical in terms of the connecting lines since the system could use existing telephone communication switching systems. However, it could be very expensive to have a separate telephone number for each IMS unit or each IMS unit location.